Lewis Acidic Boron Research Articles

Facile synthesis of bidentate boron Lewis acids by pentafluorobenzene elimination

The obtained results reveal simple synthetic approach for bidentate boron Lewis acids with the ‐OB(C6F5)2 acid group and aliphatic or aromatic linkers. Computational results showed comparable FIA for the ‐OB(C6F5)2 site in boron bidentate Lewis acids relative to initial B(C6F5)3. The relative Lewis acid strength of R[OB(C6F5)2]2 (R = C2H4, p‐C6H4, m‐C6H4) was characterized by Gutman‐Becket tests. The elimination of pentafluorobenzene by the reaction of R‐OH with B(C6F5)3 can be used for the synthesis of R‐OB(C6F5)2 derivatives and polydentate strong Lewis acids.

Boron Lewis Acids Mediated C(sp3)‐Sulfonylation of Isoindolinone Derivatives

AbstractAn efficient boron Lewis acids mediated sulfonylation reaction has been successfully developed for the synthesis of C(sp3)‐sulfonylation isoindolinones, utilizing various γ‐hydroxy lactams and sulfonic nucleophile. The prominent features of the present strategy include wide substrate scope, excellent group tolerability, high atom economy, and moderate to good yields (up to 97 % yield).

Enzyme‐Like Catalysis of Vinyl Copolymer Carrying Boron Directly Connected to Backbone: Catalytic Esterification through Cooperation of Boron with Neighboring Carboxylic Anhydride

AbstractAlternating‐rich copolymer of vinylboronic acid pinacol ester (VBpin) and maleic anhydride (MAH) was found to catalyze direct dehydrative esterification of carboxylic acid and alcohol. The key to the catalytic function is the activation of the MAH unit by the neighboring Lewis acidic boron directly connected to the backbone through the formation of five‐membered ring. The effects of the side‐chain cooperation were clarified through comparisons with the polymers having similar structures and a conventional titanium catalyst as well as the analyses of reactions with carboxylic acid or alcohol. The catalytic activity was enhanced as the molecular weight was higher, which is owing to the structural feature that boron is directly attached to the backbone. The cooperative catalysis is of interest because of its conceptual similarity with enzyme.

Enzyme-Like Catalysis of Vinyl Copolymer Carrying Boron Directly Connected to Backbone: Catalytic Esterification through Cooperation of Boron with Neighboring Carboxylic Anhydride.

Alternating-rich copolymer of vinylboronic acid pinacol ester (VBpin) and maleic anhydride (MAH) was found to catalyze direct dehydrative esterification of carboxylic acid and alcohol. The key to the catalytic function is the activation of the MAH unit by the neighboring Lewis acidic boron directly connected to the backbone through the formation of five-membered ring. The effects of the side-chain cooperation were clarified through comparisons with the polymers having similar structures and a conventional titanium catalyst as well as the analyses of reactions with carboxylic acid or alcohol. The catalytic activity was enhanced as the molecular weight was higher, which is owing to the structural feature that boron is directly attached to the backbone. The cooperative catalysis is of interest because of its conceptual similarity with enzyme.

A Highly Sterically Encumbered Boron Lewis Acid Enabled by an Organotellurium-Based Ligand.

Lewis acidic boron compounds are ubiquitous in chemistry due to their numerous applications, yet tuning and optimizing their properties towards different purposes is still a challenging field of research. In this work, the boron-based Lewis acid B[OTeF3(C6F5)2]3 was synthesized by reaction of the teflate derivative HOTeF3(C6F5)2 with BCl3 or BCl3 ⋅ SMe2. This new compound presents a remarkably high thermal stability up to 300 °C, as well as one of the most sterically encumbered boron centres known in the literature. Theoretical and experimental methods revealed that B[OTeF3(C6F5)2]3 exhibits a comparable Lewis acidity to that of the well-known B(C6F5)3. The affinity of B[OTeF3(C6F5)2]3 towards pyridine was accessed by Isothermal Titration Calorimetry (ITC) and compared to that of B(OTeF5)3 and B(C6F5)3. The ligand-transfer reactivity of this new boron compound towards different fluorides was demonstrated by the formation of an anionic Au(III) complex and a hypervalent iodine(III) species.

Magnetically Recyclable Borane Lewis Acid Catalyst for Hydrosilylation of Imines and Reductive Amination of Carbonyls.

Fluorinated arylborane-based Lewis acid catalysts have shown remarkable activity and serve as ideal examples of transition metal-free catalysts for diverse organic transformations. However, their homogeneous nature poses challenges in terms of recyclability and separation from reaction mixtures. This work presents an efficient technique for the heterogenization of boron Lewis acid catalysts by anchoring Piers' borane to allyl-functionalized iron oxide. This catalyst demonstrates excellent activity in the hydrosilylation of imines and the reductive amination of carbonyls using various silanes as reducing agents under mild reaction conditions. The catalyst exhibits broad tolerance towards a wide range of functional substrates. Furthermore, it exhibits good recyclability and can be easily separated from the products using an external magnetic field. This work represents a significant advance in the development of sustainable heterogenous metal-free catalysts for organic transformations.

Selectivity in Adduct Formation of a Bidentate Boron Lewis Acid.

A bidentate boron Lewis acid based on 1,8-diethynylanthracene has been studied in detail with respect to its adduct formation with diamines and diphosphanes of different linker lengths between the donor functions. A clear correlation between the linker length of the bifunctional base and the formation of 1 : 1 adducts, 1 : 2 adducts or oligomers was found. The adducts were characterized in solution by NMR titration experiments and structurally by X-ray diffraction. In addition, adduct formation and competition experiments of the host system with ZR3 (Z=N, P; R=H, Me) demonstrated the generally higher stability of alkylphosphane adducts compared to alkylamine adducts with boron functions. The results provide a general insight into the adduct formation of bidentate Lewis acids with guests of different sizes as well as the differences in stability between borane-amine and borane-phosphane adducts.

Formation of Strong Boron Lewis Acid Sites on Silica.

Bis(1-methyl-ortho-carboranyl)borane (HBMeoCb2) is a very strong Lewis acid that reacts with the isolated silanols present on silica partially dehydroxylated at 700 °C (SiO2-700) to form the well-defined Lewis site MeoCb2B(OSi≡) (1) and H2. 11B{1H} magic-angle spinning (MAS) nuclear magnetic resonance (NMR) data of 1 are consistent with that of a three-coordinate boron site. Contacting 1 with O═PEt3 (triethylphosphine oxide TEPO) and measuring 31P{1H} MAS NMR spectra show that 1 preserves the strong Lewis acidity of HBMeoCb2. Hydride ion affinity and fluoride ion affinity calculations using small molecules analogs of 1 also support the strong Lewis acidity of the boron sites in this material. Reactions of 1 with Cp2Hf(13CH3)2 show that the Lewis sites are capable of abstracting methide groups from Hf to form [Cp2Hf-13CH3][H313C-B(MeoCb2)OSi≡], but with a low overall efficiency.

A Water-Stable Boronate Ester Cage.

The reversible condensation of catechols and boronic acids to boronate esters is a paradigm reaction in dynamic covalent chemistry. However, facile backward hydrolysis is detrimental for stability and has so far prevented applications for boronate-based materials. Here, we introduce cubic boronate ester cages 6 derived from hexahydroxy tribenzotriquinacenes and phenylene diboronic acids with ortho-t-butyl substituents. Due to steric shielding, dynamic exchange at the Lewis acidic boron sites is feasible only under acid or base catalysis but fully prevented at neutral conditions. For the first time, boronate ester cages 6 tolerate substantial amounts of water or alcohols both in solution and solid state. The unprecedented applicability of these materials under ambient and aqueous conditions is showcased by efficient encapsulation and on-demand release of β-carotene dyes and heterogeneous water oxidation catalysis after the encapsulation of ruthenium catalysts.

Boron Lewis Acid Catalyzed Hydrophosphorylation of N-Heteroaryl-Substituted Alkenes

AbstractThe hydrophosphorylation of N-heteroaryl-substituted alkenes catalyzed by a boron Lewis acid catalyst is reported. This reaction occurs with a range of alkenes bearing N-heterocycles, including pyridines, a quinoline, a pyrrole, and a benzothiazole, resulting in the production of β-N-heteroaryl alkylphosphonates in good yields under additive-free, operationally simple conditions. The mechanistic insights suggest that this hydrophosphorylation involves the deprotonation of the P–H bond of dialkyl phosphites, enabled by the cooperative effects of a boron acid and the basic N-heterocyclic moiety of the alkenes. The resulting phosphonate anion serves as an effective phosphorus nucleophile for the conjugate addition to the concurrently formed N-protonated alkenes. These β-N-heteroaryl alkylphosphonates can be further converted into other valuable organophosphorus compounds through the introduction of an alcohol group or the reduction of the phosphine oxide moiety.

Harnessing halogen bond donors for enhanced nitrogen reduction: a case study on metal-free boron nitride single-atom catalysts.

Developing efficient catalysts for ammonia synthesis is increasingly crucial but remains a formidable challenge due to the lack of robust design criteria, particularly in addressing the activity and selectivity issues, especially in electrochemical nitrogen reduction reactions (NRR). In this study, we systematically investigated the catalytic potential of hexagonal boron nitride (BN) embedded with non-metal (C, Si, P and S) atoms as an electrocatalyst for the nitrogen reduction reaction using density functional theory (DFT) computations. The preference for non-metal-doped BN nanomaterials stems from their ability to suppress hydrogen evolution and their environmentally friendly nature, in contrast to transition metals. Among the designed single-atom catalysts (SACs), Si-doped boron nitride (SiBBN) exhibits a favorable inclination toward activating nitrogen, which is determined by the combination of advantageous molecular orbital coupling and formation of a covalent bond with the N2 molecule. Under thermal conditions, the first protonation step emerges as the rate-determining step (22.66 kcal mol-1) for SiBBN. Conversely, under electrochemical conditions, the final elementary step becomes the potential-determining step (PDS) with 2.38 eV. We explored the impact of the exogenous addition of Lewis acids (alkali metal ions, neutral boron Lewis acids, and halogen bond donors) on modulating the electrochemical NRR activity. Our results highlight the pivotal role of halogen bond donors as catalytic promoters in facilitating electron density transfer through activated N2, establishing a push-pull charge transfer mechanism that populates the distal nitrogen more than the proximal nitrogen. This facilitates the potential requirements for the first reduction step. The synergistic effect of both halogen bonding and hydrogen bonding interactions in the final reduction step was proven to be the main determinant for a significant reduction in the PDS from 2.38 to 0.10 V. Notably, this study unveils the pioneering role of halogen bond donors as promoters for NRR, providing valuable insights into the development of robust metal-free catalysts and promoters in experimental research.

Bidentate boron Lewis acids: synthesis by tin boron exchange reaction and host-guest complex formation.

Four bidentate boron Lewis acids based on the 1,8-diethynylanthracene backbone have been synthesized by a tin-boron exchange reaction with various chloroboranes, yielding the products in good to excellent yields. Complexation experiments of the host compounds with pyridine, pyrimidine and TMEDA demonstrated striking differences in terms of formation and solubility of the supramolecular adducts. The host-guest complexes were investigated by multinuclear NMR spectroscopy and structurally characterized by X-ray diffraction experiments, illustrating the adaptation of the host system upon adduct formation with different neutral guest molecules.

Activation of small molecules by ambiphilic NHC-stabilized phosphinoborenium cation: formation of boreniums with B-O-C, B-O-B, and B-O-P structural motifs.

The reactivity of the phosphinoborenium cation supported by a 1,3,4,5-tetramethylimidazolin-2-ylidene ligand toward small molecules was explored. The phosphinoborenium cation exhibited dual Lewis acid-base properties due to the presence of the Lewis acidic boron center and the Lewis basic phosphido ligand connected by a covalent bond. The reaction of the title cation with CO2 led to the insertion of a CO2 molecule into the P-B bond. The obtained borenium CO2-adduct underwent hydrolysis, forming an N-heterocyclic carbene stabilized diborenium dication bearing a B-O-B functionality. The activation of N2O proceeded via the insertion of an oxygen atom into the B-P bond of the parent cation, yielding a borenium cation with a phosphinite moiety. An alternative synthetic pathway to borenium cations with a B-O-P skeleton was achieved via the activation of secondary phosphine oxides by the phosphinoborenium cation. Furthermore, borenium cations and diborenium dications with B-O-C structural motifs were obtained from the reaction of the title compound with perfluorinated tert-butyl alcohol and hydroquinone, respectively. The structure of the obtained borenium cations is discussed based on multinuclear NMR spectroscopy, X-ray diffraction, and density functional theory calculations.

Oxazaborolidinones: Steric Coverage Effect of Lewis Acidic Boron Center in Suzuki-Miyaura Coupling Reactions.

It was demonstrated that α-hydroxycarboxamide is an excellent boron-protecting group. The reaction between α-hydroxycarboxamide and organoboronic acids produced stable oxazaborolidinones (OxBs), in which the -hybridized boron atom was sterically protected by α-hydroxycarboxamide. The alkyl groups of the α-hydroxycarboxamide moiety can dynamically cover the p-orbital of the -hybridized boron center, creating a small space around the boron atom, allowing for smooth transmetalation by a Pd catalyst and easy deprotection by water. This protecting phenomenon is effective for readily purification, Suzuki-Miyaura coupling reactions with unstable boronic acids and iterative cross-couplings.

Atom-Economical Synthesis of Lewis Acidic Boron Containing Porous Organic Polymers via Hydroboration Polymerization for Basic Chemical Capture.

Atom economy is one of the main concerns for material synthesis. Here, the facile synthesis of Lewis acidic boron-containing porous organic polymers (B-POPs) via hydroboration polymerization reaction of commercially available borane dimethyl sulfide complex (BH3 ∙SMe2 ) with multi-alkynes under mild reaction conditions is presented. This new synthetic method for B-POPs has the advantage of high atom economy. The resulted porous alkenyl borane polymers (PABPs) have unique features such as high boron content, strong Lewis acidity, and high surface areas. Owing to the strong Lewis acid-base interactions, PABPs exhibit excellent adsorptive capacity toward triethylamine (up to 841mgg-1 ) and pyridine (up to 1396mgg-1 ) vapor.

Mechanistic Insight from Lewis-Acid-Dependent Selectivity and Reversible Haloboration, as Harnessed for Boron-Based Electrophilic Cyclization Reactions.

Different reaction selectivity occurs with the Lewis acids B-chlorocatecholborane (ClBcat), B-bromocatecholborane (BrBcat), and BBr3, favoring either alkyne haloboration, electrophilic cyclization of a tethered nucleophilic sulfur onto the alkyne, or group transfer of the nucleophile. This reaction selectivity also depends on the chain length of the tethered nucleophile, revealing a subtle interplay of relative kinetics and thermodynamics. In all cases, BBr3 reacts readily with alkynes to form haloborated products; however, this process is reversible, and this reversibility can be harnessed to ultimately access regio- and stereodefined cyclic sulfonium zwitterions via the slower but thermodynamically favored electrophilic cyclization pathway. Reversibility was noted by following the reaction by NMR spectroscopy, and by characterizing the kinetic and thermodynamic products by a combination of 2D NMR spectroscopy and single-crystal X-ray diffraction. The "mixed" reagent bromocatechol borane (BrBcat) displayed reactivity between ClBcat and BBr3, producing bromoboration in some cases and electrophilic cyclization in others. With this enhanced understanding of the reaction dynamics, it becomes possible to use boron Lewis acids in a predictable manner in cases where haloboration is the kinetic product but in which the reversibility of this reaction maintains access to eventual alternative reactivity leading to desired building blocks in organic synthesis.

Tuning the Lewis Acidity of Neutral Silanes Using Perfluorinated Aryl‐ and Alkoxy Substituents

AbstractThe emerging field of Lewis acidic silanes demonstrates the versability of molecular silicon compounds for catalytic applications. Nevertheless, when compared to the multifunctional boron Lewis acid B(C6F5)3, silicon derivatives still lack in terms of reactivity. In this regard, we demonstrate the installation of perfluorotolyl groups (TolF) on neutral silicon atoms to obtain the respective tetra‐ and trisubstituted silanes Si(TolF)4 and HSi(TolF)3. These compounds were fully characterized including SC‐XRD analysis but unexpectedly showed no significant Lewis acidity. By using strongly electron‐withdrawing perfluorocresolato groups (OTolF) the tetrasubstituted silane Si(OTolF)4 was obtained, bearing an 8 % increased Δδ(31P) shift when applying the Gutmann‐Beckett method, compared to literature‐known Si(OPhF)4. Ultimately the heteroleptic Si(PhF)2pinF was successfully synthesized and fully characterized including SC‐XRD analysis, introducing a highly Lewis acidic silicon atom holding two silicon‐carbon bonds.

Lewis acid sites incorporation promotes CO2 electroreduction to multicarbon oxygenates over B-CuO nanotubes

Herein, we designed a C2+-producing catalyst by incorporating Lewis acid boron dopant into porous copper oxides nanotubes (B-CuO NTs) via a convenient electrospinning−calcination method. The B-CuO NTs catalyst achieved a 60.5% C2+ Faraday efficiency (FE) including 47% of ethanol, a 4-fold increase over CuO in a flow cell at − 0.6 V vs reversible hydrogen electrode (RHE). In situ characterizations demonstrate that the strong ability for *CO adsorption on B-CuO NTs facilitates the hydrogenation to the *CHO intermediate and promotes the C-C coupling further to *OCCHO intermediate via the proton-coupled electron transfer reactions. Theoretically calculations demonstrate that B doping induced polarized charge redistribution could suppress the *CHO transfer to C1 products by reducing the energy barrier for further OC-CHO coupling. This work provides a comprehensive understanding of Lewis acid B doping effect on regulating the C-C coupling pathway and improving the C2 selectivity.

Reactivity of a Free Aluminylene towards Boron Lewis Acids: Accessing Aluminum‐Boron‐Bonded Species

AbstractThe reactivity of the free aluminylene [N]‐Al (1) ([N]=1,8‐bis(3,5‐di‐tert‐butylphenyl)‐3,6‐di‐tert‐butylcarbazolyl) towards boron Lewis acids is investigated. A facile oxidative addition reaction of 1 with Ph2BOBPh2 furnishes an exceedingly scarce example of the free alumaborane [N]‐Al(BPh2)(OBPh2) (2) with an Al−B electron‐sharing bond. By contrast, complexation of 1 with B(C6F5)3 and HB(C6F5)2 gives rise to the corresponding Lewis adducts [N]‐Al→B(C6F5)3 (3) and [N]‐Al→BH(C6F5)2 (4), respectively, with an Al→B dative bond. Crystallization of 4 in Et2O produces the adduct [N]‐Al(Et2O)→BH(C6F5)2 (5). Quantum chemical calculations are carried out to understand the formation of 2 as well as the bonding situation of 3 and 5.

Lewis Acid-Assisted C(sp3)-C(sp3) Reductive Elimination at Gold.

The phosphine-borane iPr2P(o-C6H4)BFxyl2 (Fxyl = 3,5-(F3C)2C6H3) 1-Fxyl was found to promote the reductive elimination of ethane from [AuMe2(μ-Cl)]2. Nuclear magnetic resonance monitoring revealed the intermediate formation of the (1-Fxyl)AuMe2Cl complex. Density functional theory calculations identified a zwitterionic path as the lowest energy profile, with an overall activation barrier more than 10 kcal/mol lower than without borane assistance. The Lewis acid moiety first abstracts the chloride to generate a zwitterionic Au(III) complex, which then readily undergoes C(sp3)-C(sp3) coupling. The chloride is finally transferred back from boron to gold. The electronic features of this Lewis-assisted reductive elimination at gold have been deciphered by intrinsic bond orbital analyses. Sufficient Lewis acidity of boron is required for the ambiphilic ligand to trigger the C(sp3)-C(sp3) coupling, as shown by complementary studies with two other phosphine-boranes, and the addition of chlorides slows down the reductive elimination of ethane.